Evaluation of the intra‐ and interfractional tumor motion and variability by fiducial‐based real‐time tracking in liver stereotactic body radiation therapy

PURPOSE: Tumor motion amplitude varies during treatment. The purpose of the study was to evaluate the intra‐ and interfraction tumor motion and variability in patients with liver cancer treated with fiducial‐based real‐time tracking stereotactic body radiotherapy (SBRT). METHODS: Fourteen liver patients were treated with SBRT using a CyberKnife. Two to four fiducial markers implanted near the tumor were used for real‐time monitoring using the Synchrony system. The tumor motion information during treatment was extracted from the log files recorded by the Synchrony system. Logfile‐based amplitudes in the superior–posterior (SI), left–right (LR) and anterior–posterior (AP) directions were compared to the 4DCT‐based amplitudes. The intra‐ and interfraction amplitude variations and the incidence of baseline shifts were analyzed for 66 fractions administered to 14 patients. RESULTS: The median (range) logfile‐based liver motion amplitudes for all patients were 11.9 (5.1–17.3) mm, 1.3 (0.4–4) mm and 3.8 (0.9–7.7) mm in the SI, LR and AP directions, respectively. Compared with the logfile‐based amplitude, the 4DCT‐based amplitude was underestimated (P < 0.05). The median (range) intra‐ and interfraction liver motion amplitude variations were 4.3 (1.6–6.0) mm (SI), 0.5 (0.2–2.2) mm(LR) and 1.5 (0.3–3.3) mm (AP) and 1.7 (0.5–4.6) mm (SI), 0.3 (0.1–3.0) mm (LR) and 0.7 (0.3–2.7) mm (AP), respectively. Baseline shifts exceeding 2 mm, 3 mm and 5 mm were observed in 27.3%, 7.6% and 3% of the measurements, respectively, within 10 min, and in 66.7%, 38.1% and 19%, respectively, within 30 min for the square root of the sum of the squares of the distances in the SI, LR and AP directions (3D). The tumor motion amplitude was found to be correlated with the baseline shift. CONCLUSIONS: Most patients showed significant intra‐ and interfraction liver motion amplitude variations over the entire course of radiation. More caution is needed for patients with large tumor motion amplitudes.